Plasma arc torches are widely used in the cutting, piercing, and/or marking of metallic materials (e.g., elemental metals, metal alloys). A plasma arc torch generally includes an electrode mounted within a body of the torch (i.e., a torch body), a nozzle having an exit orifice also mounted within the torch body, electrical connections, fluid passageways for cooling fluids, shielding fluids, and arc control fluids, a swirl ring to control fluid flow patterns in a plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum (i.e., an ionized plasma gas flow stream). Gases used in the plasma arc torch can be non-oxidizing (e.g., argon, nitrogen) or oxidizing (e.g., oxygen, air).
In operation, a pilot arc is first generated between the electrode (i.e., cathode) and the nozzle (i.e., anode). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the plasma arc torch, or any of a variety of contact staring methods.
In general, the electrode, nozzle, and fluid passageways are configured in relation to one another to provide a plasma arc for cutting, piercing, or marking metallic materials. Referring to FIG. 1, in one known configuration, a plasma arc torch includes an electrode 1 and a nozzle 2 mounted in spaced relationship with a shield 3 to form one or more passageways for fluids (e.g., shield gas) to pass through a space disposed between the shield and the nozzle. In this known configuration, plasma gas flow 4 passes through the torch along the torch's longitudinal axis (e.g., about the electrode, through the nozzle, and out through the nozzle exit orifice). The shield gas 5 or other fluid passes through the one or more passageways to cool the nozzle and impinges the ionized plasma gas flow at a 90 degree angle as the plasma gas flow passes through the nozzle exit orifice. As a result of the impingement, the ionized plasma gas flow can be disrupted (e.g., generating instabilities in the plasma gas flow), which may lead to degraded cutting, piercing, or marking performance.
Referring to FIG. 2, in another known configuration, the nozzle 2 and the shield 3 can be mounted to provide substantially columnar flow of the shield gas 5 and the ionized plasma gas 4. That is, instead of impinging the ionized plasma gas flow 4 as it exits the nozzle exit orifice at a 90 degree angle, the shield gas 5 is injected out of the passageways in a parallel direction to the plasma gas flow (i.e., columnar flow) as described in U.S. Pat. No. 6,207,923 issued to Lindsay. Plasma arc torches having this configuration experience improved stability over torches that have a shield gas flow 5 that impinges the plasma gas flow 4 at a 90 degree angle. In addition, plasma arc torches that include columnar flow tend to have a large (e.g., greater than 2.4) nozzle exit orifice length to diameter ratio, L/D. Some researchers have found that a large L/D ratio will lead to the ability to cut thicker metallic workpieces and to achieve faster cutting speeds. However, in general, plasma arc torches that have substantially columnar flow of the shield gas and the plasma gas have difficulty cooling the tip of the nozzle and provide less protection from reflecting slag during cutting than plasma arc torches which use 90 degree impinging shield gas flow injection.
Thus, it would be desirable to provide a plasma arc torch which could achieve effective cooling of the nozzle and provide protection from reflecting slag while also providing a stable plasma gas flow and a large L/D ratio.